[5.3] Update README to J5.3

[heelc29]

Summary of Changes

Update some references from J5.2 to J5.3 (missed in #44010)

Testing Instructions

code review

Link to documentations

• No documentation changes for docs.joomla.org needed
• No documentation changes for manual.joomla.org needed

[Quy]

I have tested this item ✅ successfully on 99fb025

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[wilsonge]

Thanks!